This invention relates to a phase acquisition system for a local oscillator signal in a digital demodulator of the type comprising means for receiving an input signal modulated with digital data, means for multiplying the input signal with a plurality of mutually orthogonal demodulation reference signals to generate respective product signals, and means for periodically integrating the product signals separately to generate a plurality of respective sequences of integrated signals, each having an amplitude indicative of a respective portion of the digital data.
Digital demodulators of the general type described above are well known to those skilled in the art, as for example in quadrature amplitude modulation (QAM) systems. Typically, a plurality of separate component signals are combined to form a composite signal which is transmitted on a single signal channel, and each signal is orthogonal to the others and is amplitude modulated. When the received composite signal is multiplied by an appropriate local oscillator signal and the resulting product is integrated over an integral number of symbol periods, the resulting integration value is indicative of the amplitude of a respective one of the component signals of the composite signal.
Preferably, each of the component signals is sine wave modulated in order to eliminate high frequency components associated with square waves. For this reason, the local oscillator signals used as demodulation reference signals are also often sine wave modulated.
When the demodulator initially acquires an input composite signal, the frequency of the composite signal is known, but its phase is not. Data demodulation requires that the local oscillator signals which are used as demodulation reference signals must be synchronized in phase with the composite signal.
In the past, phase locked loops have been used to synchronize the reference signals with the composite signal. This approach works well in a steady state situation, where phase differences are small and conventional feedback techniques operate effectively. However, at signal acquisition the phase difference between the reference signals and the composite signal will often be large, and a conventional phase locked loop may take an excessively long time to synchronize the reference signals with the composite signal.
A need presently exists for an improved apparatus for rapidly estimating the phase of the input composite signal at acquisition of the input signal.